This invention relates generally to spectrophotometry, and more particularly concerns advancements in optical coupling to monochromators and between monochromator sections.
To facilitate understanding the invention and distinguishing it from prior art, it is necessary to define certain terms. A monochromator can be viewed as an optical filter. A double monochromator, then, is a system which causes the radiation to be twice filtered. The advantages of the double monochromator over the single monochromator are improved spectral purity and resolution. In this context, a monochromator stage is defined as a set of optical elements, including dispersing means such as a grating, necessary to receive light from one slit, render it incident to the dispersing means, and finally pass it through another slit. A double monochromator then can be either a system which passes the radiation once through two separate monochromator stages or a system which passes the radiation twice through a single monochromator stage. The latter type are frequently referred to as double-passing monochromators. The advantage of the double-passing monochromator over the two stage monochromator is largely economic. It is generally a more compact system and results in fewer essential expensive optical and mechanical components. Also, the engineering problems of tracking the two monochromator sections to the desired accuracy are substantially reduced.
A further complication exists in that in either type of double monochromator, the optics can be arranged so that the dispersion of the second section either adds to or tends to subtract from the dispersion of the first section. In the subtractive dispersion arrangement, because of the bandwidth limit imposed by the intermediate slit, the resultant total dispersion is equal to that of a single monochromator whereas the total resultant dispersion of the additive arrangement is twice that of the single monochromator. Since the amount of light flux passed for a given resolution is proportional to dispersion, for most applications the preferred embodiment is additive dispersion.
In the past, the achievement of desired monochromator efficiency has been limited by a number of factors. For example, in the case of additive dispersion double-pass monochromators, with intermediate slits located at the monochromator side of the plane or planes of the entrance and exit slits, bands of unwanted radiation are efficiently transmitted, requiring filters or selective detectors for avoiding or reducing response to such radiation bands and consequent spurious and misleading signals. See for example U.S. Pat. No. 2,922,331 to Fastie.
External slit-coupling arrangements (and some internal ones as well) have produced serious mismatch of the curvature of the exit slit of the first section as imaged upon the entrance slit of the second section. Consequently, all such arrangements described have used very short slits so that resolution is not seriously deteriorated, with the result that the "light grasp" (or "throughput," or "etendue"), i.e. the amount of light flux that can be passed for a given resolution, is seriously limited. For illustrations of these arrangements, see U.S. Pat. No. 2,922,331 and 3,567,323.